Active Electronically Scanned Array (AESA) radar systems utilizing distributed receiver/exciter elements are required to maintain synchronization (have synchronized frequency and phase) between each receiver/exciter. The signals generated at the exciters require coherence with respect to the other exciter signals to achieve proper phasing for each transmit signal. At the receiver, analog-to-digital (A/D) converter sample clocks also require coherence with respect to one another for proper reception and timing of the received signals for subsequent processing. AESA radar systems require phase lock (that is, substantially zero (0) phase drift) between receiver/exciter elements to achieve optimum performance. The effects of phase drift and/or clock jitter on radar performance results in limitations on achievable clutter cancellation performance, increased A/D noise, and other performance degradations. Minimum time requirements for AESA radar systems to maintain phase lock for coherent integration are highly application dependent and range from less than one millisecond (ms) for short range radars to many milliseconds for long range radars.
Current ultra-stable oscillators, such as rubidium oscillators or oven controlled crystal oscillators (OCXOs) for example, have a short term Allen Deviation of about (or slightly less than) 5.0×10−12 over a one second period. However, these devices have a long term Allen Deviation of only about (or slightly less than) 1.0×10−11 over a one hundred second period and about or slightly less than 4.0×10−8 per year due at least in part to aging of the crystal. As a result, in order for current ultra-stable oscillators to be used to provide coherent timing for AESA radar receiver/exciter pairs, the oscillators require synchronization with a master timing reference.
To accomplish this, conventional AESA radar systems generate and distribute a coherent timing signal by generating a timing signal from a single master oscillator and distributing the signal to receiver/exciter pairs using a series of signal splitters and phase matched interconnects, for example, cables or backplane distribution traces. Cables or backplane distribution traces may accomplish phase matching using conventional methods, such as by having cables each of whose length is calibrated to maintain phase coherence at the receiving element (e.g. identical length cables between the master timing reference and each local oscillator) to avoid phase differences among the elements. The local oscillator driving each receiver/exciter pair is in a phase-locked loop (PLL) with the master timing reference. The master timing reference adjusts the local oscillator in real time to compensate for the phase drift that naturally occurs over time between each of the local oscillators. It is contemplated that AESA radars be configured with a receiver/exciter pair per element or sub-array. This would necessitate distribution of master timing reference signals to upwards of hundreds or thousands of distributed receivers and exciters based on present AESA applications. The complexity and weight of such a distributed network would increase significantly (relative to present systems), adversely impacting designs for these already weight challenged systems. Moreover, distribution of a master timing reference in an AESA radar system implementing elements separated over great distances (e.g. such as in over the horizon (OTH) radar systems where the maximum distance between elements can exceed 2 kilometers (km)), may not be possible utilizing conventional distribution approaches. While distributing a single coherent timing signal solves the phase drift problem between the exciter/receiver pairs in a single AESA radar system, it does not enable multiple radars to remain synchronous with one another due to the drift of the single timing source. Much like the requirement to provide a single timing source to correct for the phase drift associated with each exciter/receiver pair, a similar common timing source would be required to correct for the phase drift between multiple radar systems when configured to perform coherent aperture combining such as in MIMO systems.
One approach to providing clock synchronization amongst distributed devices (e.g. cell phones) without requiring a complex distribution network involves using a global position system (GPS) disciplined oscillator to coherently synchronize local clocks (i.e. local oscillators). This approach involves receiving a one pulse per second (1PPS) GPS signal (e.g. L1 signal) and comparing the arrival time of the signal with the time code referenced to the clock on the satellite to recover the time within 100 nanoseconds deviation. While accuracy within 100 nanoseconds is sufficient for many purposes, AESA radar systems require phase matching of less than one half (½) wavelength to allow the calibration process to compensate for the phase offset between the exciter/receiver pairs with negligible phase ambiguity. Radar systems require a coherent clock reference with sufficient accuracy (e.g. phase coherence within approximately 10 picoseconds (ps) or about 10 degrees of a 1.5 GHz GPS carrier signal) to allow the radar to operate properly. Therefore, the conventional GPS approach is insufficient to replace the master oscillator and complex distribution network used on AESA radars.
An alternative approach involves measuring the precise position of each receiver/exciter pair relative to a master timing reference. The precise position measurement is utilized to determine the distance between each receiver/exciter pair and the master timing reference. A phase compensation value is generated based on the determined differential distance, and the timing signal from the master timing reference delayed to each receiver/exciter pair according to the particular phase compensation value so that the signals arrive in phase lock to the entire array. This approach suggests utilizing GPS carrier waves to measure the position of each receiver/exciter pair and the master timing reference to a very high degree of accuracy, thereby allowing for reduced length cables and/or back traces at the expense of increased complexity of an AESA radar system. Furthermore, the multitude of cables and back traces required to implement large array systems nevertheless add significant weight and cost to already overburdened array structures.
Alternative systems and methods are desired.